The invention disclosed and claimed herein is generally directed to an improved arrangement for preloading bearings, wherein the bearings are employed to support an X-ray tube target and other components for rotation. More particularly, the invention pertains to an arrangement of such type for preventing effects of thermal expansion, which occur during the production of X-rays, from interfering with bearing preload.
As is well known, modern X-ray tubes often employ tungsten electron beam targets, mounted upon rotating anodes, for the production of X-rays. The target and anode arc supported for rotation on at least two bearings, which serve as the interface between the rotating and stationary parts of the X-ray tube. As is further well known, a substantial amount of heat is usually produced in the tube when X-rays are generated. Thus, the rotary bearings must operate over a broad range of temperatures, so that individual bearing components, as well as structure adjacent thereto, are subject to thermal growth which can alter the internal clearances in the bearings. As a result, the bearings tend to become too loose or too tight, radially or axially.
In an attempt to obtain consistent bearing noise and load distribution over a broad range of temperatures, preload springs have been employed to minimize variations in bearing ball loading, as temperature changes from room temperature to the operating temperature. In order for the preload springs to function properly, one or more bearings in the system must be free to easily move, or float, axially. In the past, a preload force was typically applied to the bearing balls through the outer bearing race, or bearing ring, which was in sliding contact with the cylindrical wall of the bearing housing. However, conditions of high vacuum and temperature, generally found in an X-ray tube, tend to produce very clean surfaces which adhere to one another. If the bearing ring adheres to the adjacent housing wall, the bearing preload cannot be properly applied to the bearing balls, whereby the system loses its preload. Such situation can also occur if thermal growth eliminates all clearance between the bearing ring and the housing wall. Too little preload results in increased bearing noise, and excessive preload slows rotation of the rotary tube components, thereby reducing bearing life. Various soft metals and solid lubricants have been employed in an effort to relieve the tendency to adhere, but the outcome has been inconsistent and unreliable.